Many computer systems utilize reduced power operational modes. In the case of laptop or notebook computers, the reduced power operational modes were initially used as a mechanism to reduce battery power consumption. However, the reduced power operational modes are now also utilized on desktop computer systems, in some respects to conserve power, but in more significant respects to implement computer systems that are “instantly on”.
One of the key parameters in gauging performance of computer systems is a measure of how long the computer system takes to be fully operational. One aspect of the series of events that take place before the computer system is fully operational is execution of memory reference code. Memory reference code initializes the main memory controller in the computer system, and the memory reference code is executed just prior to passing control of the computer system to operating system programs.
Unlike early memory reference code which was written to be executed without the benefit of a stack, more recent memory reference code is written in programming languages requiring a stack (e.g., C, C++). However, at the period of time when the memory reference code is executing, the main memory is unavailable for use as a stack as the memory controller is not yet initialized. For this reason, the processor's cache is configured and utilized as a stack. To keep the memory reference code from inadvertently evicting the stack from the cache, the memory reference code is not cached; instead, the memory reference code is loaded from the relatively slow access read-only memory (ROM) directly to the processor pipelines. Based on the slow execution mode, executing the memory reference code contributes significantly to the amount of time needed to wake a computer system from a reduced power operational mode.